1. Field of the Invention
The present invention relates to a magneto-optical recording medium and reproducing method thereof to carry out recording and reproducing information by using a laser light by applying a magneto-optical effect, more specifically relates to the magneto-optical recording medium and reproducing method thereof to make a high density recording signal possible.
2. Related Art of the Invention
In magneto-optical recording being a high density, writable, recording and reproducing method, a part of layered magnetic films (a recording film structure) of a magneto-optical recording medium is locally heated to a Curie temperature or a temperature above a compensation temperature by radiating laser light, and an information signal is recorded in a predetermined part of the magnetic film contained in the recording film structure by forming a recordable magnetic domain in an external magnetic field, and this information is read by using a magneto-optical effect.
One of such magneto-optical recording system for the magneto-optical recording medium is the magnetic field modulation recording system. In this system, thermal magnetic recording is performed in a predetermined part by using an external magnetic field of which direction has been modulated according to a recording signal after raising entirely the temperature of a recording magnetic film by radiating a laser light of a certain strength using a semiconductor laser or the like. The other of the recording system is the light intensity modulation recording system. In this system, thermal magnetic recording is performed in the direction of the external magnetic field by raising the temperature of the recording magnetic film of a predetermined part by radiating the laser light which has been modulated according to the strength of the recording signal, applying the external magnetic field of a certain strength.
At the time of reproducing the recorded signal, when the laser light (reproducing light) of which polarization direction is arranged to be identical, is condensed on the magneto-optical recording medium, the direction of magnetization of recording magnetic domain is detected as the rotation of the polarization direction of the reflected light or transmitted light by a magneto-optical effect caused by the magneto-optical recording medium. By this effect, the information signal recorded is reproduced.
However, in the conventional magneto-optical recording medium, when the size of the recording magnetic domain becomes smaller than that of the light spot (reproducing-light spot) of a reproducing light on the magneto-optical recording medium, not only the recording magnetic domain to be reproduced, but also the recording magnetic domain located in front and back of the position becomes contained in the reproducing-light spot, i.e., a detection range. Therefore, some problems occur exemplified as follows: the reproducing signal becomes small to lower an S/N ratio or the reproducing signal is not outputted, because of an interference by those recording magnetic domains.
To solve these problems, a magnetic field modulation recording system using magnetically induced super resolution has been proposed to read the reproducing signal from a part of domain of the reproducing-light spot.
(I) The following is a description of a magneto-optical record reproducing system by using the magnetically induced super resolution named a double mask system which is a system of the magnetically induced super resolution.
FIG. 12 shows a configuration in reproducing by the double mask system. FIG. 12(A) is a plane view of showing a part of track of the magneto-optical recording medium 60 in the conventional double mask system. 12(B) is a sectional view showing the configuration (particularly the direction of magnetization) of a recording film structure of the magneto-optical recording medium 60.
As shown in the sectional view of FIG. 12(B), the recording film structure of the magneto-optical recording medium 60 is configured by including a reproduction layer 63, a reproduction supporting layer 64, a middle layer 65, and a recording layer 66, which are layered on a substrate (not illustrated) in order. An arrow 160 shown in the FIG. 12(B) is a movement direction along with the track of the magneto-optical recording medium 60. Arrows illustrated in respective layers 63 to 66 are show directions of magnetization in respective positions.
This conventional magneto-optical recording medium 60 requires reproducing magnetic field generating means 61 applied to the domain of the reproducing-light spot 67, initialized magnetic field generation means 62 located in the frontal position of the reproduction magnetic field generating means 61 in the movement direction 160. Hereafter, reference numerals 61 and 62 are used for a description of a reproducing operation of generated magnetic field generated by the reproducing magnetic field generating means 61 and an initialized magnetic field generated by the initialized magnetic field generation means 62, respectively. The following is the magneto-optical recording medium 60 of the double mask system configured by such manner.
First, a signal (information) is previously recorded by thermal magnetization as the recording magnetic domain 69 in the recording layer 66. Before the laser light is radiated in reproducing, the direction of magnetization of the reproduction layer 63 is arranged in the direction of the initialized magnetic field 62. At the time of reproducing, as shown in the FIG. 12(A), the reproducing laser light is radiated to the rotating magneto-optical recording medium 60 to make the reproducing-light spot 67 and raise a temperature of the recording film structure. According to this step, the distribution of temperatures as shown in the FIG. 12(A) occurs on the magneto-optical recording medium 60 to form a low temperature region 71, a high temperature region 72, and a intermediate temperature region 70.
The direction of magnetization of the reproduction layer 63 in the low temperature region 71 near a room temperature is arranged in the direction of the initialized magnetic field 62 by blocking of a exchange coupling between the reproduction layer 63 and the recording layer 66 by the middle layer 65. In the intermediate temperature region 70, the exchange coupling between the reproduction layer 63 and the recording layer 66 becomes dominant by decrease in coercive force of the reproduction layer 63 according to temperature rise caused by radiation of the reproducing laser light and also by transition of the middle layer 65 from a in-plane magnetized film having in-plane magnetic anisotropy to a perpendicular magnetized film having perpendicular magnetic anisotropy. Therefore, The direction of magnetization of the reproduction layer 63 is arranged in the direction of magnetization of the recording layer 66.
In the high temperature region 72 of the reproduction supporting layer 64 becoming a Curie temperature Tc, the exchange coupling between the reproduction layer 63, the middle layer 65, and the recording layer 66 is blocked by extinction of magnetization of the reproduction supporting layer 64 to arrange the direction of magnetization of the reproduction layer 63, of which coercive force is small, to the direction of the reproducing magnetic field 61. Therefore, a recording magnetic domain 69 is masked by both the low temperature region 71 and the high temperature region 72 inside the reproducing-light spot 67 and information can be read as a reproducing signal through the reflected light from only the recording magnetic domain 69 presented in the intermediate temperature region 70.
The direction of the reproducing magnetic field 61 is an opposite direction to the initialized magnetic field 62. After the reproducing-light spot 67 passed, the temperature of the recording layer 66 dropped again and the recording layer 66 and the reproduction layer 63 are blocked again by the middle layer 65.
According to such magneto-optical recording medium 60, even in a smaller recording magnetic domain 69 than the reproducing-light spot 67, recorded information can be reproduced with a high density without occurrence of interference by frontal and back recording magnetic domain 69.
However, the above described magneto-optical recording medium 60 has a defect in that the initialized magnetic field 62 or the reproducing magnetic field 61 are required to arrange the magnetization direction of the reproduction layer 63 to an identical direction.
Thus, a reproducing method has been proposed by using a magnetically induced super resolution to solve the above described defect.
As a method unnecessary of the initialized magnetic field or the reproducing magnetic field, a method proposed in Japanese Patent Laid-Open No. 5-81717 will be described below with reference to drawings 13(A) and 13(B). The FIG. 13(A) is the planeview showing a part of the track of the magneto-optical recording medium 80 disclosed in the above described publication and 13(B) is a sectional view showing the configuration of the recording film structure (particularly of the direction of magnetization) of the magneto-optical recording medium 80.
As shown in the sectional view of the FIG. 13(B), the magneto-optical recording medium 80 has the recording film structure containing a reproduction layer 83 and recording layer 85 formed on the substrate (not illustrated). A middle layer 84 is put between the reproduction layer 83 and the recording layer 85. The arrow 180 shown in the FIG. 13(A) shows the movement direction along with the track of the magneto-optical recording medium 80. Arrows illustrated in respective layers 83 and 85 of the FIG. 13(B) show the magnetization direction in respective positions. In the magneto-optical recording medium 80, differing from the magneto-optical recording medium 60 previously described, the magnetic film having in-plane magnetic anisotropy is used as the reproduction layer 83 at room temperature.
As same as the magneto-optical recording medium 60, the reproducing-light spot 87 is formed by radiating the reproducing laser light in reproducing information of the magneto-optical recording medium 80. When the reproducing laser light is radiated to the magneto-optical recording medium 80 during rotation, the temperature distribution of the recording film structure containing a reproduction layer 83 and recording layer 85 does not form rotation symmetry to the center of the circle of the reproducing-light spot 87. A radiated part of the reproducing-light spot 87 and the right-hand end of the back of the reproducing-light spot 87 become the high temperature region 90. The external part, included in the reproducing-light spot 87, of the high temperature region 90 becomes the low temperature region 91.
Followings are description of reproducing operation of the magneto-optical recording medium 80 configured as described above.
The recorded information is previously recorded in the recording layer 85 as a recording magnetic domain 89 smaller than the reproducing-light spot 87 by the thermal magnetic recording. The reproduction layer 83 is the in-plane magnetized film at room temperature and is the magnetic film having characteristic of becoming the perpendicular magnetized film only in the part of the high temperature region 90 inside the reproducing-light spot 87. The high temperature region 90 and the low temperature region 91 are formed by temperature rise caused by radiation of the reproducing laser light. In the high temperature region 90, the reproduction layer 83 changes to the perpendicular magnetized film and is arranged to the magnetization direction of the recording layer 85 by magnetic coupling through the intermediate layer 84. The reproduction layer 83 changes again to the in-plane magnetized film by drop of temperature caused by movement of the magneto-optical recording medium 80. Therefore, the reproduction layer 83 (the in-plane magnetized film) in the low temperature region 91 inside the reproducing-light spot 87 works as a mask and the recording magnetic domain 89 of the recording layer 85 is transferred only from the high temperature region 90 of the reproducing-light spot 87. Thus, the signal of a recording mark (the recording magnetic domain 89) smaller than the reproducing-light spot 87 can be detected.
According to the steps described above, in the magneto-optical recording medium 80, information of the recording magnetic domain 89 smaller than the reproducing-light spot 87 can be reproduced without the use of the initialized magnetic field and the reproduction magnetic field.
In the above described magneto-optical recording medium 80 by using the in-plane magnetized film in the reproduction layer 83, there is an effect of capability of making the initialized magnetic field and the reproducing magnetic field unnecessary, however, there is the following defect.
First, the direction of magnetization of the reproduction layer 83 is attracted to the direction of the magnetization of the recording layer 85 by magnetic interaction between the reproduction layer 83 and the recording layer 85 even in the low temperature region 91 masked. Therefore, an ideal surface magnetizing direction is not maintained resulting in having a vertical component of magnetization. Resultingly, transfer occurs in a domain unnecessary of transfer of the recording magnetic domain 89 to cause a deficiency of resolution and a noise in reproducing.
Second, variation of the power of the reproducing laser light (a reproducing power) changes a region, to which the recording magnetic domain 89 is transferred, to deteriorate reproducing characteristic by the wave form interference of a transfer domain, because a critical temperature, in which the reproduction layer 83 changes from the in-plane magnetized film to the perpendicular magnetized film, is constant.
In addition, change of an ambient temperature such as the temperature in a drive and the like requires change of setting of reproducing power. However, in the case where particularly the ambient temperature rises, requires reducing the reproducing power to decrease in temperature difference between the critical temperature and the room temperature capable of transfer of the signal of the recording layer 85. As the result, the region (the low temperature region 91) masked by the reproduction layer 83, that has become the in-plane magnetized film, reduces. Therefore, there are problems in which the reproducing signal is deteriorated by decrease in resolution and the signal transfer in the recording layer 85 becomes insufficient.
In other words, the high temperature region varies according to variation of the above described variation of reproducing power and the above described variation of ambient temperature.
(II) On the other hand, in the magneto-optical recording medium by using the above described magnetically induced super resolution system, a rare earth-transition metal alloy is mainly used for the magnetic layer.
FIG. 27 shows the vertical component of a magnetic moment of the sub-lattice of the transition metal in the magneto-optical recording medium by using the magnetically induced super resolution system based on the conventional art. Arrows 1300 and 1400 in a first magnetic layer 1100 and a second magnetic layer 1200 represent the vertical components of the magnetic moments of the sub-lattices of the transition metals of the first magnetic layer 1100 and the second magnetic layer 1200, respectively.
The first magnetic layer 1100 is the in-plane magnetized film at room temperature and changes from the in-plane magnetized film to the perpendicular magnetized film according to temperature rise. The second magnetic layer 1200 is a film consisting of such as TbFeCo and DyFeCo and having a large perpendicular magnetic anisotropy at room temperature. Recorded information is kept depending on upward or downward direction of the magnetizing domain of this second magnetic layer 1200 toward the surface of the film.
When a light beam is radiated from the substrate side to the magneto-optical recording medium with the above described configuration, a temperature gradient occurs in a beam spot 1700 to present a region of a high temperature and a region of a low temperature. In this condition, the first magnetic layer 1100 does not contribute to pole Kerr effect to the utmost because it becomes the in-plane magnetized film and the recorded information stored in this second magnetic layer 1200 is masked to disappear, in the region of the low temperature in the beam spot 1700.
On the other hand, in the region of the high temperature in the beam spot 1700, the magnetically induced super resolution is realized by that the first magnetic layer 1100 becomes the perpendicular magnetized film to cause a magnetostatic coupling with the second magnetic layer 1200. Therefore, Information stored in recorded in the second magnetic layer 1200 is transferred to the first magnetic layer 1100.
As a whole of the beam spot 1700, the magnetically induced super resolution is realized by that recorded information in the second magnetic layer 1200 is transferred to a smaller region in comparison with the size of the beam spot 1700, because of a part masked by the first magnetic layer 1100.
The magneto-optical recording medium by using such magnetically induced super resolution system can satisfy requirement of high density by a narrowed track. In the above described configuration, a transition region occurs as an intermediate condition to change from the in-plane magnetized film to the perpendicular magnetized film in the beam spot 1700. Namely, a whole film does not change abruptly from the in-plane magnetized film to the perpendicular magnetized film in a predetermined temperature, but a certain range of temperature becomes transition region.
In the transition region, the first magnetic layer has not become a perfect in-plane magnetized film and perfect masking of the recorded information kept by the second magnetic layer is impossible. On the contrary, the first magnetic layer is not a perfect perpendicular magnetized film in the transition region and a magnetostatic coupling force with the second magnetic layer is small to be difficult to yield a large signal.
Therefore, the first magnetic layer in the transition region cannot mask sufficiently the recorded information kept by the second magnetic layer to increase cross talk from an adjacent track. In addition, the first magnetic layer in the transition region has a weak magnetostatic coupling force with the second magnetic layer to make sufficient transfer of the recorded information from the second magnetic layer impossible.
Furthermore, the first magnetic layer is frequently prepared by using a material, a rare earth metal generally expensive. This makes the cost of material high in the case where the magnetic layer is prepared thick and productivity is worsen. Thus, a cheap magneto-optical recording medium is difficult to be provided.